You created a user or multiple users that should not have access to a publication, although the rest of the user group should?
Open the Portfolio Manager and go to the overview of your publications by clicking on 'Content' > 'Contents' in the top menu or 'Contents' in the quick overview.
With the filter menu at the left you can search for the relevant publication. Once found, you can click on the dropdown of that publication and choose 'User blacklist':
A pop-up will appear, where you can select the user(s) that should not have access to this publication in the app: